RESOLVE, Multishot Echoplanar Diffusion Weighted Imaging
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Scans were acquired at 1.5 T with a typical diffusion weighted echoplanar (EPI) scan sequence (A), and also with a RESOLVE work-in-progress (WIP) scan sequence (B). RESOLVE is a multi-shot technique that uses 2D navigator correction with readout-segmented EPI (ISMRM 2004, p442). Axial scans of the brain in a normal volunteer, at the level of the petrous apices, are presented. The conventional single shot sequence was acquired (with complete coverage of the brain) in 0:54 min:sec. The RESOLVE (multishot) scan was acquired in 3:02 min:sec. A parallel imaging factor of 2 was used for the single shot acquisition, with no parallel imaging employed with the multishot acquisition. Partial Fourier was employed in (A), resulting in a shorter scan time but also some image blurring when compared to (B).
Imaging Findings:
The RESOLVE sequence permits acquisition of multishot diffusion weighted echoplanar scans. This feature can be used in many ways, with a simplistic but very clinically relevant application illustrated. In particular at 3 T, bulk susceptibility artifacts can markedly degrade diffusion-weighted images, rendering them non-diagnostic. Acceptable image quality has been achieved to date at 3 T by using parallel imaging. This technique can decrease the impact of the susceptibility differences, which otherwise lead to substantial artifacts, in particular at the interface between the brain and air filled sinuses. The two scans illustrated show that multishot EPI (RESOLVE in this instance) can be used in the same manner as parallel imaging, with a similar (if not somewhat greater, in the current example) impact on bulk susceptibility artifact. By applying RESOLVE in the future in combination with parallel imaging, a further reduction in bulk susceptibility artifacts should be readily achieved (ISMRM 2006, p1046), bringing scans at 3 T in line with the quality of 1.5 T scans, at least in terms of the prominence of this artifact. RESOLVE will likely find application in a wide range of DWI and DTI examinations, which require diffusion-weighted images with a higher level of detail than that offered by current single-shot EPI scans.
Acknowledgment: these scans were acquired in collaboration with David Porter of Siemens Healthcare, Erlangen, Germany.
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